The re-entry of a vehicle into the atmosphere poses many problems which are difficult to overcome. For example, the re-entry of high ballistic parameter vehicles, which may be characterized by fairly sharp slender cones, encounters a serious problem of vehicle versus decoy discrimination. For weapon system applications, it is necessary to make the re-entry vehicle difficult if not impossible to observe to overcome the enemy defense system. Since a vehicle cannot be made completely invisible to defense systems, the use of decoys is important to increase the vehicle penetration capability. A serious problem in this respect occurs due to boundary layer transition.
A nonsuction re-entry vehicle with, for example, a length of 7 ft., will undergo boundary layer transition at an altitude of approximately 90,000 to 100,000 ft. A small nonsuction decoy would undergo boundary layer transition at a much lower altitude, perhaps 50,000 ft. When a re-entry vehicle undergoes transition, there is a resultant change in re-entry vehicle drag and hence in the vehicle velocity. The smaller decoy would not experience this change until it reaches a lower altitude. This differnce in relative velocity results in the so-called metric discriminant.